KHARKIV, Ukraine (AP) – Russian authorities say a truck bomb has caused a fire and the collapse of a section of a bridge linking Russia-annexed Crimea with Russia. The bridge is a key supply artery for Moscow’s faltering war effort in southern Ukraine. The speaker of Crimea’s Kremlin-backed regional parliament immediately accused Ukraine, though the Kremlin didn’t apportion blame. Ukrainian officials have repeatedly threatened to strike the bridge and some celebrated the explosion, but Kyiv stopped short of claiming responsibility. Saturday’s attack on the bridge comes a day after Russian President Vladimir Putin turned 70. It deals him a humiliating blow that could lead him to up the ante in his war on Ukraine.